1. Field of the Invention
The present invention relates to digital signaling systems and more particularly to controlling the signal settling time of digital signaling systems.
2. Description of the Related Art
In computer and information processing systems, various integrated circuit chips must communicate digitally with each other over common buses. The signal frequency at which this communication occurs can limit the performance of the overall system. Thus the higher the communication frequency, the better. The maximum frequency at which a system communicates is a function not only of the time that it takes for the electromagnetic wavefronts to propagate on the bus from one chip to another, but also of the time required for the signals to settle to levels that can be reliably recognized at the receiving bus nodes as being high or low, referred to as the settling time.
The length of the settling time is a function of the amount of reflection and ringing that occurs on the signal bus. The more effective the termination of a bus system, the smaller the effects of reflection and ringing in the system and the shorter the overall settling time of the signal.
It is known to terminate a bus using a number of different schemes. Three types of termination schemes are referred to as a Scheme-1 Termination scheme, a Scheme-2 Termination scheme and a Scheme-3 Termination scheme. With a 3-node system having a transmission line characteristic impedance of Z ohms everywhere on the line, a middle node is defined as the node having the shortest distance from the intersection of the transmission lines, and the remaining two nodes are defined as end nodes. The middle buffer is defined as the buffer directly associated with the middle node.
A Scheme-1 Termination scheme is characterized as follows. When it is receiving signals, each of the end node output buffers acts as a pull-up terminator (i.e., the end of the transmission line is coupled to the upper power rail via a resistance that is equal to the characteristic impedance of the line) and the middle node output buffer is tri-stated (i.e., functions as an open circuit). When it is driving signals, the output buffer of an end node pulls up through Z ohms and pulls down through Z ohms; the middle node output buffer pulls up through Z ohms and pulls down through Z/2 ohms.
A Scheme-2 Termination scheme is characterized as follows. The output buffer at each node acts as a pull-up terminator when it is receiving signals. When it is driving signals, each node""s output buffer pulls up through Z ohms and pulls down through Z/2 ohms.
A Scheme-3 Termination scheme is characterized as follows. With the Scheme-3 Termination Scheme, the transmission lines are coupled to a network of resistors at the intersection rather than being directly connected. The output buffer at each node acts as a pull-up terminator when receiving signals. When it is driving signals, each node""s output buffer pulls up through Z ohms and pulls down through a very low resistance to achieve large voltage swings.
It is desirable to determine which termination scheme provides better signaling characteristics, such as minimum settling time, for a particular system geometry. It is also desirable, if constrained to use a particular scheme, to determine how to adjust the lengths of the transmission lines so as to minimize settling time.
It has been discovered that a Scheme-1 Termination scheme will maximize system signaling rates by minimizing the signal settling-time when the distance from the middle node to the intersection of the transmission lines is much smaller than the sum of the distances from the end nodes to the intersection of the transmission lines. If the distance from the middle node to the intersection of the transmission lines is not much smaller than the sum of the distances from the end nodes to the intersection of the transmission lines, it has been discovered that a Scheme-2 Termination scheme will minimize the signal settling-time.
Alternately, when a Scheme-3 Termination scheme is available, the Scheme-3 Termination scheme maximizes system signaling rates by minimizing the signal settling time when the third transmission line length is more than the second transmission line length divided by two. Otherwise a Scheme-1 Termination scheme minimizes the signal settling time.
More specifically, in one aspect, the invention relates to a method of selecting a termination scheme between a first scheme and a second scheme to minimize signal settling time in a three node system having first, second and third transmission lines connected at a junction. The method includes: determining a first transmission line length of the first transmission line; determining a second transmission line length of the second transmission line; determining a third transmission line length of the third transmission line, the first transmission line length being greater than the second transmission line length and the second transmission line length being greater than the third transmission line length; selecting the first termination scheme if the third transmission line length is short relative to the combination of the first transmission line length plus the second transmission line length, otherwise selecting the second termination scheme.
In another aspect, the invention relates to a method of selecting a termination scheme from a first termination scheme, a second termination scheme and a third termination scheme to minimize signal settling time in a three node system having first, second and third transmission lines connected at a junction. The method includes: determining a first transmission line length of the first transmission line; determining a second transmission line length of the second transmission line; determining a third transmission line length of the third transmission line; selecting the first termination scheme if the third transmission line length is less than half of the second transmission line length otherwise selecting the third termination scheme.
In another aspect, the invention relates to a method of adjusting line lengths in a three node system to minimize signal settling time. The method includes: determining whether a first termination scheme, a second termination scheme or a third termination scheme is used by the three node system; determining a first transmission line length of a first transmission line; determining a second transmission line length of a second transmission line; determining a third transmission line length of a third transmission line, the first transmission line length being greater than the second transmission line length and the second transmission line length being greater than the third transmission line length; adjusting, when the first termination scheme is used, the first transmission line length, the second transmission line length and the third transmission line length to minimize the sum of two times the first transmission line length plus four times the third transmission line length; adjusting, when the second termination scheme is used, the first transmission line length, the second transmission line length and the third transmission line length to minimize the sum of three times the first transmission line length plus the second transmission line length; and, adjusting, when the third termination scheme is used, the first transmission line length, the second transmission line length and the third transmission line length to minimize the sum of the first transmission line length plus the second transmission line length. These formulas for the effects of such adjustments on settling time assume that, following such adjustments, the first transmission line length remains greater than the second transmission line length, and the second transmission line length remains greater than the third transmission line length.